To determine if a number is divisible by 7, take the last digit off the number,
double it and subtract the doubled number from the remaining number. If the
result is evenly divisible by 7 (e.g. 14, 7, 0, -7, etc.), then the number is
divisible by seven. This may need to be repeated several times.

Example: Is 3101 evenly divisible by 7?

310 - take off the last digit of the number which was 1
-2 - double the removed digit and subtract it
308 - repeat the process by taking off the 8
-16 - and doubling it to get 16 which is subtracted
14 - the result is 14 which is a multiple of 7